The size of wireless communication systems in terms of the number of nodes they include have increased rapidly in recent years. One type of communication system where this trend is especially strong is the cellular system.
One reason for this growth is the reduction in size of some system nodes, such as base stations. Base stations can nowadays be fairly small and cover a small area. This allows great flexibility in the systems but also puts high demands on them.
As the size of the base stations decrease there is desire to make them as simple as possible in order to keep the costs down. However, they must still have a first-rate functionality.
One area where this is evident is the issue of clocks and timing.
A base station used in a wireless communication system needs a stable frequency reference to be able to communicate with mobile stations at a reasonable bitrate. Demand for higher bitrates is increasing over time, depending of new end user features that requires large amount of data to be transferred to and from the mobile stations.
This leads to the requirement of more bandwidth and higher frequencies, which emphasises the need for exact timing.
This exact timing can in many instances be obtained using an oscillator that is heated and kept at a specific temperature with the help of a temperature regulation device, such as an Oven.
Yet another solution is to use a Global Positioning System (GPS) receiver for time and frequency synchronization of the local oscillator.
The use of GPS is for instance described in U.S. Pat. No. 7,606,541.
Stable oscillators are costly and require volume/space in the device. Warming the oscillator will first of all require the additional arrangement needed for keeping a certain temperature, which adds to the cost and the complexity. However, it will also draw extra energy, which even further raises the costs. The time until a stable frequency can be generated is further rather long depending of time sample integrating and oscillator warming, which means that valuable time may be lost before the required accuracy is obtained.
When synchronization is solved with the help of a GPS, the obvious drawback is that the GPS signal is not always available, typically not in indoor systems. Furthermore, there is here a requirement of additional receivers for receiving GPS signals.
As can be seen, there is therefore a need for alternative ways of obtaining accurate timing of a base station.